1. Field of the Invention
The present invention relates to service request handling in IMC (IP Multimedia Core). In particular, the invention relates to redirecting a service request for a served user in IMC.
2. Description of the Related Art
There are many applications of the Internet that require the creation and management of a session, where a session is considered an exchange of data between an association of participants. The implementation of these applications is complicated by the practices of participants: users may move between endpoints, they may be addressable by multiple names, and they may communicate in several different media—sometimes simultaneously. Numerous protocols have been authored that carry various forms of real-time multimedia session data such as voice, video, or text messages. The Session Initiation Protocol (SIP) works in concert with these protocols by enabling Internet endpoints (called user agents) to discover one another and to agree on a characterization of a session they would like to share. For locating prospective session participants, and for other functions, SIP enables the creation of an infrastructure of network hosts (called proxy servers) to which user agents can send registrations, invitations to sessions, and other requests. SIP is an agile, general-purpose tool for creating, modifying, and terminating sessions that works independently of underlying transport protocols and without dependency on the type of session that is being established.
More details about SIP as defined by the Internet Engineering Task Force are described in Request For Comments 3261. As mentioned above, SIP allows the establishment, handling and release of end-to-end multimedia sessions. There are several additions to the SIP protocol, which e.g. allow event notification based on SIP, which is the basis for a SIP based Presence Service and other services.
The 3GPP IMS (Third Generation Partnership Project IP Multimedia Subsystem) utilizes SIP in order to achieve a wide range of functionality within the 3GPP (wireless) network.
S-CSCFs (Serving Call State Control Functions) in the IMS download filter criterions (FCs) from a HSS (Home Subscriber System). FCs are evaluated one-by-one, i.e. an incoming request is checked by a terminating S-CSCF based on the public user identity in the Request-URI (Universal Resource Identifier) as to whether the first or initial FC (highest priority) matches. If it matches, the S-CSCF sends it to the related Application Server (AS) that is indicated by the FC and adds a “dialog identifier” in the route header that is pointing back to the S-CSCF.
When the request is sent back from the AS and received again at the S-CSCF, the S-CSCF identifies the request by the dialog identifier and checks for matching of the next following FCs of lower priority, and applies the filter criteria on the SIP method as received from the previously contacted AS. Depending on the result of the previous process, the S-CSCF may contact one or more application server(s).
Due to some special services the AS may redirect the request (e.g. Call Forwarding). In these cases, it might be undesirable for the AS that the S-CSCF performs the subsequent FCs. According to the prior art, such behavior of the S-CSCF cannot be affected by the AS.
Call forwarding as an example of service request redirection is one of the most generally used services in telecommunication systems. Its utilization significantly impacts session handling in IP Multimedia Core Subsystem, thus it should be accurately defined.
The latest version of 3GPP TS 24.229 Release 5 standard (v. 5.3.0) specifies terminating procedures at S-CSCF generally without considering the special effects of executed services. Request redirection modifies the Request-URI data of the affected session, which requires special processing in S-CSCF which cannot be accomplished with the general description of the standard. According to the prior art as described in chapter 5.4.3.3 “Request terminated at the served user” of 3GPP TS 24.229, v. 5.3.0, in a situation of request redirection such as call forwarding, the execution of the rest of procedures will conflict with the purpose of call forwarding.
According to the general description in point 11 of chapter 5.4.3.3 “Request terminated at the served user” of 3GPP TS 24.229, v. 5.3.0, the Request-URI is overwritten, eliminating the possibility of call forwarding itself. In other words, according to the prior art, a Request-URI is built by the S-CSCF with the contents of a saved Contact URL (Universal Resource Locator) where the user served by the S-CSCF is reachable, which Contact URL is determined from the destination public user identity.
A specific description of call forwarding is not given so far in IP Multimedia Subsystem (IMS), and call forwarding handling in IMS is completely different from that in other existing systems.